CRITERIA OF ADEQUACY FOR A GRAMMAR

There is an old saying, “If you stand for nothing, you’ll fall for everything”.  How do you decide whether the set of rules that you’ve proposed for some set of sentences in a language is the right one, or the best one, given that there are an infinite set of possible grammatical descriptions?

In formulating a grammar, or a set of rules that we assume models the language user’s knowledge of language, we must first decide how we are going to evaluate the grammar that we propose.  Chomsky (1965) proposed three criteria of adequacy for a grammatical description, which he dubbed:

(i)             observational adequacy;

(ii)           descriptive adequacy;

(iii)          Explanatory adequacy.

I will now discuss these concepts.

A.   Observational Adequacy

Linguists who formulate grammars of languages that are not their own, and who work with native speakers of those languages, are called field linguists.  They typically start by finding out the words for various concepts in the target language, and eventually ask the speaker if (s)he can put the words together in this way or that way to form an acceptable sentence in the language.  After collecting the responses for some time period (say, an hour-and-a half, for example),   the field linguist leaves and analyzes the responses, trying to figure out the rules that generate all of the acceptable strings and none of the unacceptable strings.  A set of rules, or grammar, that achieves this, is said to be observationally adequate.  Hence,   observational adequacy can be defined as follows:

Observational adequacy:  the ability of a grammar to generate all and only the grammatical sentences of a language in a fixed body of data (called a corpus).

B.     Descriptive Adequacy

As we saw from the Jabberwocky example,  natural languages are infinite,  and hence a grammar of a natural language must be able to  generate an infinite number of sentences.   To take the example of the field linguist above,  after the field linguist has formulated a grammar that is observationally  adequate,  he tests the grammar against the native speaker’s intuitions by asking the native speaker if some further set of sentences that are not in the original corpus are acceptable sentences in the language.

If a grammar generates all and only the set of grammatical sentences in the language,  it is said to be descriptively adequate.

Descriptive adequacy:   the ability of a grammar to generate all and only the grammatical sentences of the language.

However,   we are not only interested in generating the right set of strings.  Remember,  what we are really interested in modeling is the full set of abilities that native speakers  have,  and one of those abilities is the ability to recognize the meanings of sentences.  For example,  we know that The cat is on the mat does not mean that John saw Mary.   We therefore have to build in this ability as well.   A traditional way of describing a grammar is as an infinite set of pairings of form and meaning.  Let us therefore revise our definitions of observational and descriptive adequacy as follows:

Observational adequacy (Final Version):  the ability of a grammar  to generate all and only  the  grammatical sentences of a language in a fixed body of data (called a corpus),  and to pair each grammatical sentence with its meaning.

Descriptive adequacy(Final Version):   the ability of a grammar to generate all and only the grammatical sentences of the language,  and to pair each grammatical sentence with its meaning.

C.     Explanatory Adequacy:

The third requirement is not,  strictly speaking,  a requirement on grammars,  but,  rather,  a requirement on the account that underlies the construction of a particular grammar,  i.e.  an account of what a possible grammar of a human language can be.   This needs a little more explanation.

When we formulate a grammar,  we must have,  at some level,  a set of assumptions as to what a possible grammar can be- there are certain possibilities for rules that don’t even occur to us.  We therefore have,  if only  implicitly,  a theory of possible grammars.

It is commonplace to view the task of a linguist, in discovering a descriptively adequate grammar of a language, as being identical to the task of a child,  who is trying to discover the descriptively adequate (adult) grammar of the language of her or his community.  Because linguists are trying to model  the abilities of native speakers,    one of their goals is to try to formulate this theory,  called a theory of universal grammar,  as well as the grammars of particular languages.  We would therefore say that the relation of the theory of grammar  to grammars of particular languages could be described as follows:

Theory of Grammar (Universal Grammar) ={G₁,…., G_n}

In other words,  a theory of grammar is a specification of the possible grammars,  which we’re calling G₁ through G_n (instead of French,  English,  Ewe,  Chinese,  etc.).  Now,  a theory of grammar  that is the correct account of what a possible grammar of a natural language  should predict only  the set of actual possible grammars,  and should not predict that some grammar is the grammar of a natural language that is never realized in fact. In other words,  a theory of  grammar should not over-predict.

There is a distinction between “actual grammars of human languages” and “possible grammars of human languages”.   As  Chomsky & Halle put it in the preface to their classic work in phonology,  The Sound Pattern of English (Chomsky & Halle (1968)),  “If a nuclear explosion were to wipe out everybody on earth except for the inhabitants of Tanzania,  we would not want to say that pitch is a linguistic universal.”  (the assumption being that the language spoken in Tanzania is what is known as a pitch-accent, or  tone,  language).  External circumstances would cause only one language to spoken in the world,  the speakers of all of the others having been wiped out by nuclear extinction,  but the speakers of  that language would have the capacity to learn other languages that are not pitch-accent languages.

Remember, in constructing an account of what a possible human language is,   we are modeling actual human capacities,  and the assumption is that humans will only consider a  certain set of grammars as possible grammars as human languages.  Explanatory adequacy,  therefore,  can be defined as a requirement that a theory of grammar only allow for the possible grammars of human languages.

Explanatory Adequacy= the ability of a theory of grammar to predict only the set of possible grammars of human languages.